Interactions between atoms in condensed matter result in properties that are characteristic of bulk solids. Bulk solids are classified as large particles or crystallites that are multiple tens of nanometers or larger in size. Classic scientific fields of study including physics, chemistry, and materials science that are used to explain the physical, mechanical, optical, etc., properties of bulk solids require the use of quantum mechanics to explain observed phenomena such as chemical bonds, superconductivity, electron spin and magnetic properties of matter, radiant heat emission, or radioactive decay.
As the length scale in these bulk solids approach a very small size, e.g., <50 nm (where nm=nanometer, i.e., 10−9 meter), these materials exhibit changes in properties that diverge from those in the bulk state. Particles in this size range can be referred to as nanocrystals. These changes in properties are the result of a reduction in electron energy levels. For example, small nanocrystals of gallium nitride (GaN), referred to as quantum dots, have been shown to have a photoluminescence peak centered at 2.95 eV (electron volts), which is 0.5 eV below the bulk GaN bandgap (B. Daudin et al., MRS Internet J. Nitride Semicond. Res. 4S1, G9.2 (1999)). These quantum dots trap electrons in a point comprised of a tiny cluster of inorganic semiconductor material <30 nm in diameter. Many investigators believe that quantum dots will provide a variety of advances for electronics: increased efficiency, reduced power consumption, increased speed of operation and novel electronic characteristics (M. May, Science Observer, July-August (1996)). A challenge that exists is to develop general processes for creating these small nanocrystals at the required size scale.
When these small nanocrystals are organized in a manner such that interparticle separations are on the order of 0.5–10 nm, energy transfer between neighboring nanocrystals and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity (C. B. Murray et al., Annu. Rev. Mater. Sci., 30, 545–610 (2000)). At separations <0.5 nm exchange interactions result in semiconducting, metallic, or superconducting properties in assemblies that are normally insulating in the bulk. A major challenge in this area is to develop a general procedure for creating superlattices of nanocrystals (an ordered, periodic array of nanocrystals) having the desired interparticle separation necessary to produce desired materials properties.
Most of the nanocrystal work mentioned in the literature centers around inorganic/ionic materials (C. B. Murray et al., IBM J. Res. & Dev., v45, No. 1, pp 47–56, January 2001). The assembly of atoms to generate superlattices of such inorganic nanocrystals requires specific methods of synthesis and functionalities added to surfaces to induce self-assembly. Generation of self-assembled superlattices of inorganic nanocrystals has been reviewed in the literature (C. B. Murray, et al., Ann. Rev. Mat. Sci., 30, 545–610 (2000). Typically, fabrication of self-assembled nanocrystals starts with atomic deposition onto a surface of a semiconducting substrate, with the deposited material having a smaller bandgap than the substrate (P. Petroff, Physics Today, 54(5), 46–54 (2001)). Though this and other similar methods, which are high vacuum processes, such as molecular beam epitaxy (MBE) or chemical-vapor deposition (CVD) are currently practiced, they require exacting control of deposition parameters. Typically, the procedures described for creating self-assembled superlattices of inorganic nanocrystals require very slow solvent evaporation under controlled conditions, as rapid evaporation of the solvent will result in an amorphous aggregate.
Organic compounds are defined as molecular compounds containing the element carbon with covalent bonds. Such compounds are most often isolated or synthesized from petroleum, coal, vegetable, or animal sources, as well as synthesized from other organic, carbonate, or cyanide compounds (R. T. Morrison and R. N. Boyd, Organic Chemistry 3rd edition, Allyn and Bacon Inc., Boston, 1, (1976)). Though the number of classes of organic molecular materials is significantly greater than inorganic compounds, the literature related to formation of organic molecular nanocrystals is limited. One example of the formation of organic molecular nanocrystals previously described relates to dye compounds that form H- or J-aggregates. The number of monomer units associated with H- and J-aggregate nanocrystals has been estimated to be ca. 4 monomer units per absorbing unit (A. Herz, Photog. Sci. Eng., 18, 323–335 (1974)). Interactions among dye molecules can generate large spectral shifts and/or changes in spectral band shape and intensity in absorption spectra. The magnitude and the direction of these shifts are determined by the internal structure (i.e., H- or J-aggregate structure) of the nanocrystal. It is known that nanocrystals of certain dyes can be generated by gradually increasing their concentration in solution, and the internal structure of the nanocrystal is identified by the gradual shift of the absorption spectra to shorter wavelength (in the case of H-aggregates) or a sudden shift to longer wavelengths (as in the case of J-aggregates) (E. Jelley, Nature, 138, 1009–1010 (1936)). These H- and J-aggregate nanocrystals exhibit unique properties that differ from the properties of the bulk solid, and are used, e.g., in silver halide based photographic products.
Precipitation from liquid solvents is regarded as a general process for generating crystals of organic molecular materials. An analogous, general process for generating nanocrystals of organic materials is precipitation from compressed fluids such as CO2 by the rapid expansion of supercritical solutions (RESS) techniques such as described in U.S. 2003/0054957 A1. Nanoscale particles resulting from such process may exhibit multiple molecular packing structures that are the result of rapid depressurization leading to rapid desaturation of a compressed fluid that contains an organic molecular material. A fundamental difference between precipitation from liquids and precipitation from compressed fluids such as CO2 by the RESS process is the significantly faster rates of supersaturation generation and dissipation (D. Matson et al., Ind. Eng. Chem. Res., 26, 2298–2306 (1987)). Hence, precipitation from compressed fluids such as CO2 is a convenient process for generating organic nanocrystals.
An important challenge in the study of organic nanocrystals is the ability to create self assembled organic nanocrystal superlattices. High vacuum techniques used for creating superlattices of inorganic nanocrystals are not easily adapted to deposition of most organic or polymer materials (S. Forrest, MRS Bulletin, Jan.Feb., 108–112, (2001)). Further, deposition on large substrates using high vacuum techniques is difficult due to practical limitations of chamber size as well as problems with proper substrate alignment and deposition pixellation. Techniques such as spin casting or spraying from liquids, which are sometimes used to create superlattices of inorganic nanocrystals, are limited by the solvent evaporation process. After coating, the solvent must be removed, which hinders the ability to make defect-free coatings. Spin-casting techniques are also limited in terms of scalability, due to a limit in the substrate size that can be accommodated in a spin-cast apparatus.
Langmuir-Blodgett (LB) film techniques have been shown to be of use for deposition of organic and polymer materials, especially for the deposition of molecularly self assembled materials. However, it is not clear that such LB techniques would be useful in generating self assembled superlattices of organic nanocrystals (the distinction being that organic nanocrystals comprise a cluster of organic molecules, and hence the superlattice is a self assembled structure where each individual entity is a cluster of molecules). Further, scaling LB methods for large-scale applications is prohibitive due to technical considerations including the need for clean-room environment and functionalized surfaces which are material specific. Cost considerations are also an issue in trying to scale up LB deposition. A LB deposition apparatus built at the University of Connecticut (C. Mirley et al., Langmuir, 10, 230–2375 (1994)), e.g., was capable of dipping the substrate to be coated at a rate up to 0.13 m (meters) per minute, and the device had to be housed in a class 100 clean room. At the United States Department of Energy Pacific Northwest National Laboratory, a state of the art KSV-5000 Langmuir-Blodgett Instrument has been configured in a class 1000 clean room which is capable of dip coating a substrate at a rate up to 0.80 m per minute. The maximum substrate size that can be coated is 0.1 m×0.1 m.
In contrast, in conventional roll-roll coating operations (e.g., offset printing), flexible substrates are coated at speeds that are faster than the state of the art LB techniques by more than three orders of magnitude. Further, the width and the length of the substrate coated in these roll-to-roll operations are also typically significantly larger (e.g., width by at least an order of magnitude and the length by 4–5 orders of magnitude).
Hence, there is a need for a process that would allow for the rapid generation of organic molecular nanocrystal superlattices. In particular, there exists a need for a simple, robust and rapid coating procedure for the generation of self assembled, superlattice thin films of organic molecular nanocrystals, that can be easily adapted and scaled to a large number of materials, surfaces, environments and deposition conditions. Such a procedure can find application in many technologies and industrial products requiring thin films of organic materials such as display technologies and display products that can include liquid crystal (LC) displays and organic light emitting diode (OLED) displays.
In the case of charged colloidal suspensions, it has been observed that crystallization may be induced by deionizing the suspension to an ionic strength below about 0.001 M (Y. Monovoukas and A. P. Gast, J. Colloid Interface Sci., 128, 533–548, (1989). The general procedure used here is again very slow, requiring the use of ion exchange resin to reduce the ionic strength of the suspension. Hence, once again, there is a need for process that would allow for the rapid generation of organic nanocrystal superlattices.